These studies have concerned estrogen induction of the biosynthesis of the egg yolk phosphoproteins, phosvitin and lipovitellin, in the rooster. It has been demonstrated that these proteins are synthesized in the liver and transported in the blood as a high molecular weight precursor complex, vitellogenin. An in vitro synthesis system has been developed for use as an assay to follow purification of the messenger-RNA specifying vitellogenin. This messenger-RNA has been purified more than 600-fold from a crude preparation of liver polysomes. Purified preparations of vitellogenin messenger-RNA will allow the synthesis of radioactive complementary DNA, which can be used as a probe to study changes in the regulation of transcription of liver chromatin, in vitro, in response to steroid hormones.